The present invention relates to mixtures of aqueous polymer dispersions and to a process for bonding substrates using these mixtures.
In the automotive industry and particularly in car construction, a continuously increasing number of parts for the interior trim of motor vehicles, such as side parts, instrument panels, covers and shelves, are being made of PVC-film-laminated fiberboard moldings and, more recently, of thermoplastics. In the lamination process (vacuum forming), resilience forces, and, in some cases, extremely strong resilience forces, occur due to the locally high elongation of the laminating film. These forces are transmitted from the adhesive to the substrate. Under extreme climatic conditions, a laminated interior part must withstand high temperatures for prolonged periods without delamination. Another problem in the production of laminated interior parts is the migration of plasticizer from the PVC film material which can clearly reduce the strength of the bond over a period of time. To date, the high property profile for bonds of this type has only been achieved by solvent-containing two-component polyurethane systems.
For reasons of pollution control, industrial hygiene and safety in processing, polymer dispersions, particularly polyurethane dispersions, are being increasingly used for this purpose.
Adhesive compounds based on aqueous polymer dispersions are known. Homopolymers and copolymers of acrylate and vinyl acetate monomers are widely used for adhesive compounds. These latices are modified with isocyanate reactive monomers to obtain high-quality, crosslinkable adhesives or coatings.
German Auslegeschrift 2,643,642 describes the modification of an acrylate dispersion containing 0.1 to 100 parts by weight (based on dispersion solids) of an isocyanate. The dispersion serves as a contact adhesive compound. Another example of an isocyanate-modified polymer dispersion can be found in Japanese Kokai No. 75/69137 which describes the production of a waterproof adhesive compound of an aqueous polymer dispersion (such as a butadiene/methylmethacrylate latex), a polyvinyl alcohol and an isocyanate component in an apolar solvent.
Japanese patent application 79/102334 describes an adhesive compound containing a water-soluble polymer or aqueous polymer dispersion and a hydrophilic compound containing at least two isocyanate groups per molecule. Cellulose derivatives or polyvinyl pyrrolidone for example are used as the water-soluble polymers while ethylene/vinyl acetate and vinyl acetate/acrylic acid copolymers are used as the polymer dispersions. The hydrophilic component consists of a reaction product of a diisocyanate and an isocyanate-reactive polyether. The adhesive compound shows high resistance to water.
Japanese patent application 69/11279 describes an adhesive compound of high bond strength and high resistance to ageing and to chemicals. The compound in question consists of a mixture of a polymer dispersion containing carboxylate groups with an isocyanate-reactive copolymer and is intended in particular for the coating of textiles.
U.S. Pat. No. 4,396,738 describes an aqueous adhesive compound and coating compound of an aqueous dispersion of a polymer or copolymer and an aqueous dispersion of a polyisocyanate at least partly reacted off with a monofunctional alcohol.
U.S. Pat. No. 4,433,095 describes an adhesive compound of an aqueous polymer dispersion and a water-dispersible aromatic polyisocyanate having an isocyanate functionality of more than 2.2. Mixtures of dispersions are not described in detail.
U.S. Pat. No. 3,931,088 describes the production of adhesive compounds containing an aqueous solution of a polyvinyl alcohol, an aqueous emulsion of a vinyl acetate polymer and/or an aqueous latex of a butadiene polymer and also a solution of an isocyanate compound or an isocyanate polymer.
U.S. Pat. No. 4,636,546 describes a process for the production of coatings and adhesive compounds in which aqueous dispersions of polymers containing isocyanate-reactive hydrogen are first reacted with a water-emulsifiable polyisocyanate and are then mixed with an aqueous polyurethane dispersion. The necessary preliminary reaction means that the process involves an additional step.
U.S. Pat. No. 4,762,880 describes an adhesive compound for a heat laminating process which consists of a dispersion of an aromatically substituted polyurethane and an emulsifiable polyisocyanate. The laminating process takes place at high temperatures of the order of 150.degree. C. which can result in damage to the substrate and to the laminate.
European patent 276,482 describes mixtures of aqueous polymer dispersions and their use as coating compounds and coatings. The claimed mixtures are distinguished by improved fastness to wet rubbing, a more pleasant feel and improved ironability on such substrates as fabrics or leather. However, the films formed only soften at high temperatures which are too high for heat laminating processes with thermoplastics, such as ABS.
The problem addressed by the present invention was to provide adhesives having a reduced activation temperature and improved adhesion, particularly for ABS (i.e., acrylonitrile/butadiene/styrene).